Part II of my answers to Brendan's questions covers basic changes to the combat rules.
5. Initiative: individual, group, or something else?
Based on surprise, Dex, and weapon length.
6. Are there critical hits and fumbles? How do they work?
7. Do I get any benefits for wearing a helmet?
Helmets (and shields) shall be splintered!
8. Can I hurt my friends if I fire into melee or do something similarly silly?
Yes (5+ on d6 if you miss your target.)
I don't use standard initiative at all. Based on my research, initiative was originally just a roll to see which player goes first, with the actual order of effects based on weapon length. I've been mostly going on weapon length only: first strike to the longer weapon, longer weapon also goes first when fighting animals or other tooth-and-claw attacks, but otherwise shorter weapon goes first. I'm going to change that a bit, keeping "longer weapon gets first strike," but otherwise going in Dex order (substituting Move for Dex for most monsters.) Weapon length is now a tie breaker.
I don't do "crit on 20, fumble on 1". Instead, I'm only using critical hits or fumbles in specific situations (magic-user wielding a two-hand sword in close formation? OK, but on a missed attack, roll a d6 to see if you hit a friend.) The firing into mêlée example in question #8 is one such "fumble".
I mentioned in passing at the beginning of our local games that I will be using "Shields Shall Be Splintered", but no one's tried it. I should make it more explicit in a written handout, and mention that helmets, like shields, can be sacrificed to block damage. This decision must be made before damage is rolled, and some attacks may prevent such a response (helmets are ineffectual against snakes biting your leg, for example.)